Positron emission tomography (PET/CT) is a nuclear medicine scan used in medical diagnosis. Nuclear medicine imaging utilizes small quantities of radioactive material in the diagnoses and determination of disease severity or treatment of various types of diseases including neurological, endocrine, gastrointestinal, heart, cancer disorders, among other body abnormalities. Because molecular activity can be pinpointed through nuclear medicine, the procedures involved allow for the potential of early disease detection and subsequent immediate response to the necessary interventions (Guiberteau, 2016).
In most medical center, superimposition of images is done with CT or MRI to create special images. These images give information from two separate exams which are correlated and then interpreted into a single image that has information that is more precise and also diagnosis that is accurate. Additionally, PET/CT scans are able to do imaging exams simultaneously. The PET/CT scan does body functions measurements such as oxygen utilization, blood flow, glucose metabolism, which help doctors in the accurate evaluation of the functioning of tissues and organs (Wehrl, Judenhoffer, & Wiehr, et al., 2009). CT imaging utilizes specialized x-ray equipment with contrast materials where necessary to give several images of the internal workings in the body.(See Appendix 1 for basic architecture and flow chart of PET/CT scanner). Radiologists interpret these images using a monitor. CT scans offer anatomic information in a precise manner. The PET scanner has low specificity but high sensitivity while the CT scanner has high specificity and low sensitivity hence a combination of the two offers the best image resolutions for specific diagnoses.
The use of PET/CT scans in diagnosis of cancer is known to result in therapeutic decision changes in 30% to 40% of cancer cases (Hillner, 2009).The characterization and diagnosis by MRI and CT imaging is premised on different criteria such as tissue attenuation, texture, and size. MRI and CT offer information on tissue density, organ size, and also precise topographic and spatial localization. On the other hand, the basis of PET imaging is the radioactive agent bio-distribution over space and time which enables the visualization of pathophysiological and physiological processes of disease functionality characterization.
Risks of a PET scan
When the PET is used in combination with a CT scan, additional tracers are needed and this can prove to be harmful to persons with a history of kidney diseases or persons with high levels of creatinine from medications they may be currently taking. When the eGFR > 45 it is an indication that there is no increased risk of damage to the kidney form the contrast material while an eGFR > 30, but does not surpass 45 indicates a slight risk of kidney damage. In such a situation, injecting additional fluid into the veins prior to and after injecting the contrast material will effectively prevent renal damage to the patient(Zagoria, 2015)
Other minor risks include discomfort for persons who are afraid of needles or enclosed places (claustrophobic). There is also the possibility of developing an allergy due to the tracers. A person should alert the physician of any such allergies prior to having the test done. The Physician will require the patient to answer a number of questions to determine their eligibility (see Appendix 2 for the questionnaire)
Benefits of PET/CT
PET/CT scans offer tremendous advantages which can be summarized as: precise monitoring and treatment; accurate localization and staging; and early disease diagnosis. With the high quality images, patients have a better chance of getting precise diagnosis and avoid any additional and unnecessary procedures. A PET/CT scan allows for early cancer detection and reveals tumours which could be obscured by scarred tissue resulting from radiotherapy or surgery especially in the neck and head area (Fleming & Johansen, 2008).
Previously, difficulty was experienced in the interpretation of CT scan results done at different locations and at different times than a PET scan mainly because the body position of the patient changed with each scan. A PET/CT scan provides a complete image of the body occurrences both metabolically and anatomically simultaneously (RSNA, 2014)
The ability of the scan to study the body functions allows for detection of any anatomical changes making it an effective diagnosis tool. The scan is able to distinguish between malignant and benign tumours which reduce the possibility of unnecessary surgeries. It is also effective in diagnosing the early stages of Alzheimer's disease, dementias, epilepsy, and other neurological disorders. PET/CT scans are infection free and pose no danger of secondary infection for patients (Glaudemans & Signore, 2010).
The Future of PET/CT Scans
Molecular imaging using PET/CT scans is pivotal in cancer management as it assists in choosing the radiotherapy and cytostatic procedure that is most appropriate. It also contributes to the recurrence early detection. PET/CT shows promising signs of improving personalized medicine through better characterization of tumour extent, biological features, and response (Oyen, 2007).
The use of intra-operative probes aid in minimal invasive surgery in tumour and sentinel nodes removal which could present morphological alteration that is unremarkable. Additionally, PET/CT offers treatment that is efficient through target radiotherapy of neuroendocrine tumours, thyroid disease, pain palliations for diffuse bone metastases patients, as well as non-
There are new approaches being investigated which utilize alpha particles (Lucignani, 2008). In addition, PET/CT scans use in defining volumes of biological tumours and radiotherapy dose painting is promising plans for more efficient but less toxic tumour control (Weber et al., 2008; Anderson & Ferdani, 2009)
Positron emission tomography (PET) is now a vital imaging tool for cancer diagnosis and staging, as well as presenting prognostic evidences based on response. PET is important for the evaluation of unspecified solitary pulmonary nodules or masses, where PET has confirmed to be significantly more precise than computed tomography (CT) in the distinction between malignant and benign lesions. In the examinations for metastatic spread, PET is a vital in conventional imaging. However, currently, PET does not exchange conventional imaging.
Benefits of PET/CT scan for cancer diagnosis
Due to nuclear medicine image inherent characteristics as well as their low power resolution, it becomes challenging to define the exact disease' anatomical location which further complicates the interpretation of results. To avoid this challenge, a combination of the functional and molecular imaging offered by PET and the CT provided anatomical imaging is merged using combined scanners (Townsend, 2008) while development is underway for PET/MRI prototype (Wehrl et el., 2009).These hybrids allow for a diagnostic procedure to be carried out singularly through structure and function evaluation. The introduction of combined imaging allows for the possibility of diagnostic process re-examination, order of studies performance, and also therapeutic pathway construction.
A radionuclide in a PET scanner tracks at a molecular level, a precise biological process. The radioactive tracer is able to distinguish between molecular/cellular activities that are normal from the abnormal. The positron emitters commonly used in this process include nitrogen-13, oxygen-15, carbon-11, and fluorine-18. These emitters while in their non-radioactive form are all active molecule biological constituents. Fluorine-18 is most suited for hydrogen. The emitters are thus suitable for labelling any molecule without making any changes to the respective metabolic pathway (Bockisch, Freudenberg, Schmidt, et al., 2009).
Tumour Growth Process
The simple way of describing the process of a tumour growth is that they divide and multiply before infiltrating neighbouring tissues and structures and finally spreading to distant locations through a process referred to as metastasis. Tumours require energy for growth and metastasis and hence utilize glucose. Glucose provides the tumour cells with the necessary energy for activity. While the normal body cells also utilize glucose to carry out their respective functions, tumour cells utilize glucose at higher rates than the normal cells (Salskov et al., 2007). Labelling is done using fluorine-18 which like FDG is a glucose analogue. Fluorine-18 is used as a tracer as it decays quickly which minimizes exposure of the patient to radiation. Fluorine-18 is also preferred as a tracer because it naturally indicates the metabolic state of cells and more so in cancer cells and hence it is detected easily. For the detection of cancerous cells using PET/CT scan, Fluorine-18 is often used in the form of the biologically active F18-FDG model which is a radioactive labelled glucose analogue which allows for metabolism of glucose in both abnormal and normal cells (Salskov et al., 2007)
In this case study, the patient was a 39 year old female who had a documented history of colorectal cancer. She was first evaluated in April 2015. After the initial evaluation, the patient went through preoperative radiation therapy. This was followed by rectal cancer surgical resection which was done in late May 2015. Systematic chemotherapy course was adhered to after the surgical procedure. In September 2015, a follow up PET scan showed a previously identified uptake of FDG in the pelvic area which was consistent with the malignancy that had been treated. The respective lung lesion pathology results indicated adenocarcinoma that was metastatic and which was consistent with primary colorectal cancer. As mentioned earlier, a follow PET/CT scan that was done some months after the resection indicated that there was no recurrence f cancer or existence of any residual malignancy as shown in the images below. A follow up was scheduled for July of 2017 and is likely to show no evidence of malignancy.
PET/CT after resection showing no evidence of malignancy
The role of FDG-PET was critical in the patient's initial staging of colorectal cancer and also in the monitoring that followed. In addition, PET/CT also aided in the characterization of the SPN lesion as being suspicious for cancer.
Physicians and professionals in the medical field have recognized and appreciated the role that PET/CT scans play in managing solitary metastatic lesions and more so in the determination of whether a patient is a candidate for a metastasectomy or lymph node, liver, or lung lesion ablation using radiotherapy. In the case where the metastasis is solitary, the outcomes have shown to be promising as seen in the case study discussed above. In this case, the nodule was characterized using PET/CT scan as being suspicious for cancer while it was still at a relatively small size and at an initial stage of metastasis. By using PET/CT scan, the SPN lesion was confirmed as being solitary and eligible for a procedural metastasectomy. However, it should be noted that nodules that are less than a centimetre in size cannot be characterized reliably by a PET/CT scan.
Anderson, C.J., Ferdani, R., (2009). Copper-64 radiopharmaceuticals for PET imaging of cancer: advances in preclinical and clinical research, Cancer Biother Radiopharm. 24: 379-93.
Bockisch, A., Freudenberg, L.S., Schmidt, D., Kuwert, T., (2009). Hybrid imaging by SPECT/CT and PET/CT: proven outcomes in cancer imaging. Semin Nucl Med. 39: 276-89.
Fleming AJ, Jr, Johansen ME. (2009). The clinician’s expectations from the use of positron emission tomography/computed tomography scanning in untreated and treated head and neck cancer patients. Curr Opin Otolaryngol Head Neck Surg. 16:127–34.
Glaudemans AWJM, Signore A.(2010). FDG-PET/CT in infections: the imaging method of choice? European Journal of Nuclear Medicine and Molecular Imaging. 37(10):1986–1991
Guiberteau, M (2016) Positron Emission Tomography - Computed Tomography (PET/CT)
Lucignani, G.,(2008). Alpha-particle radioimmunotherapy with astatine-211 and bismuth-213,Eur J Nucl Med Mol Imaging 35; 9:1729-33.
Macklis, R.M., (2007). Radioimmunotherapy as a therapeutic option for Non-Hodgkin's lymphoma, Semin Radiat Oncol. 17:176-83.
MacManus MP, Seymour JF, Hicks RJ. (2007). Overview of early response assessment in lymphoma with FDG-PET. Cancer Imaging.7:10–8
Oyen, W.J., et al. (2007). Targeted therapy in nuclear medicine--current status and future prospects. Ann Oncol. 18:1782-92.
RSNA (2014). FDG-PET/CT UPICT V 1.0 Imaging Protocol.
Salskov, A., Tammisetti, V.S., Grierson, J., Vesselle, H., (2007). FLT: measuring tumor cell proliferation in vivo with positron emission tomography and 3'-deoxy-3'-[18F]fluorothymidine, Semin Nucl Med. 37;6:429-39.
Townsend, D.W., (2008). Dual-modality imaging: combining anatomy and function, J Nucl Med.49:938-55.
Weber, D.C., et al., R.,(2009). Recurrence pattern after [(18)F]Fluoroethyltyrosine-Positron Emission Tomography-guided radiotherapy for high-grade glioma: A prospective study, Radiother Oncol. 93;3:586-592.
Wehrl, H.F., Judenhofer, M.S., Wiehr, S., Pichler, B.J., (2009). Pre-clinical PET/MR: technological advances and new perspectives in biomedical research, Eur J Nucl Med Mol Imaging 36:56-68.
Zagoria, R. (2015). CT and MRI Contrast and Kidney Function. (Retrieved 19th April, 2017).